Modem control device for the engine management in motor vehicles must increasingly include functions for the so-called start-stop operation, such as the selective shutdown and restarting of the combustion engine during waiting phases at traffic lights. In order to reliably predict the starting ability of the combustion engine at all times and in every operating state of the motor vehicle, the engine management must know load state of the battery with sufficient accuracy.
The load state of the battery is usually modeled in the engine management from the recordable variables battery voltage, battery current and battery temperature, with the aid of a battery model. The accuracy of the prediction of the starting ability is a function of the battery voltage, in particular. Consequently, it is especially important to record the battery voltage as precisely as possible.
FIG. 1 shows a known device for recording the mentioned input variables battery voltage UBAT, battery current IBAT and battery temperature TBAT for the battery model in engine-control devices.
The battery voltage is available via the connection of the positive pole UBR of the battery, which is required in control device 1 anyway and switched via primary relay 2. With the aid of voltage dividers R1 and R2, this voltage signal is adapted to the input voltage range of the associated analog-digital converter ADC of microcontroller μC used in control device 1. C1 together with R1 and R2 forms a low-pass for signal filtering.
The battery current is recorded with the aid of an active sensor 3, which supplies a voltage signal IBAT which is equivalent to the battery current. If the maximum amplitude of this voltage signal is above the input voltage range of associated analog-digital converter ADC, a voltage divider R3 and R4 is likewise utilized here to adapt the voltage level. If the maximum amplitude of IBAT does not exceed the input voltage range of analog-digital converter ADC, resistor R4 may be omitted. C2 together with R3 and R4 forms a low-pass for signal filtering.
The recording of the battery temperature is implemented with the aid of an NTC thermistor 5 in battery 4. The associated evaluation circuit has a capacitor C3 and resistors R5 and R6. C3 together with R5 and R6 forms a low-pass for signal filtering. One connection of R5 is connected to 3.3V or 5V. This is the input voltage range of the following analog-digital converter ADC. As an alternative, instead of the NTC thermistor, an active sensor may be used as well, which provides a voltage signal TBAT that is equivalent to the battery temperature. Its analysis is then implemented analogously to the illustrated circuit for battery current IBAT.
If—as illustrated in FIG. 1—the signal of switched positive battery pole UBR is utilized, which is available in control device 1 as it is, the following disadvantages result:                Due to line impedances, the potential of control device pin UBR is not identical with the actual potential of the positive battery pole. These potential differences are especially dependent on the design of the vehicle cable harness and on the loading of the vehicle electrical system, i.e., on the different operating states of the additional users connected to the vehicle electrical system, which are indicated by reference numerals 17a, 17b and 17c in FIG. 1. Consequently, the potential differences are unable to be precisely determined and the result be taken into account in the engine management.        The afore-described potential differences in recording the potential of the positive battery pole could be avoided or at least considerably reduced depending on the design of the vehicle cable harness, if control device pin UBD at which the so-called permanent positive is connected is used instead of control device pin UBR. The control unit receives a much lower current via this pin, so that considerably lower potential differences occur as a result of fluctuations in the loading of the vehicle electrical system. However, in the switched-off state of the engine-control device, voltage divider R1 and R2 would then lead to a significant increase in the quiescent current drawn from the battery. With typical dimensioning of voltage splitter R1 and R2, this increase in the quiescent current is unacceptable.        For monitoring purposes, positive battery pole signal UBR must be adapted to the input voltage range of analog-digital converter ADC with the aid of voltage divider R1 and R2, in such a way that even brief voltages considerably above the rated value of the battery voltage are still able to be detected. However, the large voltage range resulting from this requirement is in contradiction to the most precise possible detection of the battery voltage for the start-stop operation. The detection of the battery voltage in the range close to its rated value of typically 14 V is sufficient for this purpose.        In the related art illustrated in FIG. 1 it is assumed that the potential of the negative pole of the battery is identical to the potential of control device ground GND. However, considerable potential differences may result here due to cable harness impedances. These potential differences are a function of the design of the vehicle ground connection and also of the loading of the vehicle electrical system, i.e., on the different operating states of the vehicle, and falsify the recording of the actual potential of the negative battery pole.        